The name of a file. All files have names. Different operating systems impose different restrictions on filenames. Most operating systems, for example, prohibit the use of certain characters in a filename and impose a limit on the length of a filename. In addition, many systems, including DOS and UNIX, allow a filename extension that consists of one or more characters following the proper filename. The filename extension usually indicates what type of file it is.
Within a single directory, filenames must be unique. However, two files in different directories may have the same name. Some operating systems, such as UNIX and the Macintosh operating system, allow a file to have more than one name, called an alias. (In Unix, aliases are called links or symbolic links.)